Rapid expansion in use of portable electronic equipment such as notebook computers, mobile phones, camcorders and digital cameras has led to increased demand for secondary batteries having a high-energy density which are used as power sources for such electronic equipment and also demand for secondary batteries for electric vehicles (EVs). As a typical example of such secondary batteries, a lithium secondary battery comprises an anode of a carbonaceous material, a cathode of a lithium metal oxide, a separator of a polyolefin material and a non-aqueous electrolyte.
The electrode of the lithium secondary battery is generally fabricated by coating an electrode slurry on metal foil. In this connection, the electrode slurry is prepared by mixing an electrode mix, which is usually composed of an electrode active material for storage of energy, a conductive material for imparting electrical conductivity and a binder for binding the electrode foil with the conductive material, in a suitable solvent such as NMP (N-methyl-2-pyrrolidone).
Various kinds of materials are often added to lithium secondary batteries, depending upon desired purposes and applications. For example, inorganic materials may be typically added as such additives. The present invention achieves improvement in battery lifespan characteristics by adding a small amount of alumina having a specified particle diameter to a cathode mix.
In this regard, some prior arts, which employ alumina as an electrode component, will now be reviewed hereinafter.
Korean Patent Registration No. 508920 discloses a lithium-sulfur secondary battery utilizing elemental sulfur and sulfur-based compounds as a cathode active material, instead of lithium transition metal oxides. According to this technique, battery capacity and lifespan characteristics are improved via addition of 1 to 50% by weight of alumina having a particle size of less than 35000 nm, based on the total weight of the cathode mix. This patent states that desired improvement in battery performance due to addition of an additive cannot be obtained when alumina is added in an amount of less than 1% by weight.
Korean Patent Laid-open Publication No. 2005-14189 discloses a technique for decreasing capacity reduction and improving cycle characteristics upon charge/discharge of a lithium secondary battery, by adding 1 to 3% by weight of Al2O3 and 5 to 10% by weight of LiCo3 to an anode active material composed of tin and carbon. This patent does not specifically describe a particle diameter of alumina (Al2O3) and just shows that addition of less than 1% by weight of alumina cannot result in increases of battery capacity.
Japanese Patent Registration No. 3245886 discloses a technique of decreasing performance deterioration of a battery due to the presence of the residual alkali upon high-temperature storage, which involves adding 0.002 to 0.2 moles of SiO2, Al2O3 and the like to a cathode active material of LiCoO2, based on 1 mole of the cathode active material, thereby decreasing an amount of residual alkali. This Japanese Patent does not specifically reveal a particle diameter of alumina used therein, and it is believed from working examples thereof that problems associated with deterioration of battery performance cannot be solved with addition of such additives below the above-specified content ranges. Further, this Japanese Patent also states that desired high-temperature storage characteristics are achieved with addition of such additives to a cathode active material, instead of an anode active material.
Korean Patent Laid-open Publication No. 2000-0056301 discloses a technique of alleviating non-uniformity of electrolyte distribution via addition of 1 to 5% by weight of a metal oxide, for example alumina to an anode mix, thereby improving decreasing occurrence of localized swelling and unreacted parts of an anode over charge/discharge cycles. Likewise, this Korean Patent also does not mention about a particle diameter of the metal oxide used therein and describes that absorption effects of the electrolyte solution are poor upon addition of less than 1% by weight of the metal oxide.
As discussed above, it can be seen that some of conventional arts have confirmed some desired effects via addition of alumina to electrodes and the like. However, according to these conventional arts, alumina is illustrated only as an example of metal oxides which are added to electrodes, or is used in conjunction with other materials. Further, none of those arts discloses technical details stemming from inherent properties of alumina itself related to the particle diameter.
Further, according to conventional arts, it is described that desired effects are exerted only when at least 1% by weight of alumina is added if it is used alone. However, when large quantities of additives, which are not electrode active materials that are directly responsible for absorption (intercalation) and desorption (deintercalation) of lithium ions in lithium secondary batteries, are added as described above, it is impossible to overcome the fundamental limitations associated with decreases in energy density of the batteries even though other general properties may be improved.